1. Field of the Invention
The present invention relates to diffraction type optical displacement encoders. More particularly, the present invention relates to improvements in an optical displacement encoder for detecting a positional relationship between two members from a change in a photoelectrically transduced signal caused by a relative displacement between a main scale formed thereon with an optical grating and an index scale formed thereon with an optical grating corresponding to that of the main scale.
2. Description of the Prior Art
In the field of measuring a feed rate and the like of a tool in a machine tool, there is widely used an optical encoder wherein a main scale formed with a first grating is affixed onto one of opposing members, an index scale formed with a second grating, a lighting system including a light source and a light-receiving element are affixed onto the other, and a detection signal that varies periodically in accordance with a relative movement between the opposing members is produced.
The conventional diffraction-type optical displacement encoder uses a collimated lighting system in general, whereby the first grating and the second grating are equal in pitch.
In contrast thereto, there has heretofore been proposed a diffraction-type displacement encoder wherein the pitch of the second grating is 1/n (n is a natural member) of the pitch of the first grating, as shown for example, in Japanese Patent Application No. 61-191532.
Such an optical encoder having an even number n pitch grating, as depicted in FIG. 11, mainly comprises: a collimated lighting system 10 including a light-emitting diode (LED) 12 having an effective wave length .lambda., a collimator lens 14, a main scale 16 formed with a first grating 18 of a pitch P, an index scale 20 spaced V (gap) apart from the first grating 18 and formed with a second grating 22 of a pitch Q=P/(2n) (n is a (natural number), a light-receiving element 24 for photoelectrically transducing a light emitted from the collimated lighting system 10 and filtered through the first and the second gratings 18 and 22, and a preamplifier 26 for amplifying a output signal therefrom to obtain a detection signal(a).
A S/N ratio of the detection signal (a) is normally represented by a ratio of PP/DC between an amplitude PP and a DC component DC. An example of the experimental result in a case of a pitch Q=P/2 and, when a grating gap (V) is varied, is indicated by a solid line A in FIG. 12.
Since the S/N ratio (=PP/DC) of the detection signal (a) is fluctuated by the grating gap (V) as apparent from FIG. 12, if the index scale 20 is fixed at a position where PP/DC is at the minimum at the time of assembling the encoder, then the S/N ratio of the detection signal (a) is lowered so that its resistance to noise is diminished. This presents a problem in that the positioning accuracy becomes severely critical and the encoder is thus increased in cost.
Diffraction optical displacement encoders are typically of either the transmission type or the reflection type. The former, as shown in FIG. 11, detects light transmitted through the main scale 16, whereas the latter or reflection type detects light reflected by the main scale. In the reflection type, a light emitting element and light-receiving element are provided on the same side with respect to the main scale so as to facilitate its assembly and use in machine tool devices or systems.
FIG. 15 shows an example of the reflection type encoder utilizing the conventional collimated lighting rays and comprises: an emitting diode 12 as being the light source, a collimator lens 14, a main scale 16 formed with a periodical first grating 18, a light-transmitting index scale 20 formed with a periodical second grating 22 corresponding to the first grating 18 of the main scale 16 and is movable relative to the main scale 16, and a light-receiving element 24 for photoelectrically transducing reflected rays R from the collimated lighting system. Rays R are reflected by the first grating 18 of the main scale 16 and transmitted through the second grating 22 of the index scale 20, whereby a periodical detection signal is thereby produced in accordance with a relative displacement between the main scale 16 and the index scale 20.
However, the use of the collimated lighting rays requires a large collimating lens 14 having high accuracy, whereby the encoder becomes large-sized in a thickness direction (D), and suffers from the additional problems that methods of affixing and positioning the elements are difficult to perform.
A solution to this problem has heretofore been proposed, for example in applicant's Japanese Patent Application No. 61-194183, in which a diffusive light source is used as depicted in FIG. 16.
This reflection type encoder comprises: a laser diode (LD) tip 34 as being the diffusive light source (a point light source), a main scale 16 formed with the periodical first grating 18, a light-transmitting index scale 20 formed with the periodical second grating 22 corresponding to the first grating 18 of the main scale 16, and a light-receiving element 24 for photoelectrically transducing light reflected from the first grating 18 of the main scale 16 and transmitted through the second grating 22 of the index scale 20, whereby a periodical detection signal is thus produced in accordance with a relative displacement in a direction X between the main scale 16 and the index scale 20.
Moreover the LD tip 34 is typically housed in a container 32 provided with a monitor light-receiving element for example.
Distances between the LD tip 34 and the first grating 18 and between the second grating 22 and the first grating 18 are set at u and V, respectively, and pitches of the first grating 18 and the second grating 22 are set at P and Q, respectively. Further, when the most practical arrangement of u=V is adopted (as proposed in Japanese Patent Application No. 61-194183) and if Q=2P (adopted for example from the similarity), then a detection signal having a satisfactory S/N ratio can be obtained.
Further, in a coherent case where the point light source (LD tip 34) has a coherence, even under Q=P, a detection signal can be obtained due to the diffraction effect (as proposed in Japanese Patent Application No. 61-194184).
Additionally, it is also clear that when under Q=2P/m (m is a natural number), a detection signal is generally obtainable as discussed in Japanese Patent Application Nos. 61-208554 and 61-208555 which were also proposed by the present applicant.
As described above, the reflection type encoder, in which the point light source (34) is used as it is, is effective in rendering the encoder small-sized in the thickness direction (D).
However, in the use of the laser diode, although the LD tip 34 itself is small in size, the container 32 for the LD tip 34 is relatively large considering the amount of radiant heat and the like. More particularly, when the arrangement of u=V, it is difficult to place the light source and the light-receiving element 24 close to each other, and, moreover, the encoder cannot be made very small in size in a direction parallel to a plane where the graduation (the first grating 18) of the main scale 16 is formed.
Furthermore, it is necessary to support the point light source obliquely, and normally, it is necessary to provide a plurality of pairs of second gratings 22 and light-receiving elements 24 in association with phases of 0.degree. , 90.degree. , 180.degree. , 270.degree. , etc. of the detection signal. However, the encoder of this type suffered from the problem that methods of arranging and supporting the above members are difficult to perform.